An exposure drum provides a movable support surface for a lithographic plate, film, or other sensitized material, which is secured to the surface of the drum. The drum is rotated while the sensitized printing plate is imaged to form a particular image on the plate. The image may be formed by various techniques, such as a scanning LED array.
A prior system for securing sensitized material to a drum surface is shown in U.S. Pat. No. 4,899,214. The patent describes an exposure drum having grooves machined into the drum surface. A vacuum is drawn through the drum and grooves to hold the sensitized material onto the drum surface. This system is designed for use with sensitized material of a single size thus, aside from a lead edge groove, requiring only one groove pattern. If, however, sensitized material of a smaller size were placed on the drum surface then no substantially air-tight seal would be formed between the groove pattern and the sensitized material, thereby preventing the formation of a vacuum to secure the material onto the drum.
One proposal suggests machining a multitude of grooves onto the drum surface to accommodate differently sized plates. However, three problems arise with this proposal. First, the cost of machining the precision grooves is prohibitively high. Second, when a large sized plate is secured to the drum, several other sets of grooved patterns, adapted for securing smaller sized plates to the drum, lie beneath the portion of the plate where the image is being formed. Although no vacuum is applied to these other grooves, a vacuum is still formed within these grooves, causing the plate portion immediately above the grooves to be pulled nonuniformly against the mask. As a result, the image formed using such a system will be out of focus in the regions of the underlying grooves. Third, some groove patterns may cross each other, thus creating a sealing problem at the cross-over points.
The alternate method of replacing drums for each different plate size would be costly and impractical. Because of the exposure drum's mechanical complexity, considerable time and labor is required to replace the drum.
Therefore, prior systems are unable to adequately provide for the mounting of various sized plates onto the exposure drum.